I can't find Farro (Triticum dicoccum) where I live and Spelta (Triticum spelta) is relatively common.
How interchangeable are they? What should I expect if I make a Farro dish using Spelta?
According to Heidi Julavits' in the New York Times Sunday Magazine, the grains are very different in their cooking properties, even though they are both members of the wheat family, and despite claims that they can be used in the same recipes:
You should look for recipes specifically for Spelt, if this is what you have available.